There has been proposed an achromatic lens system that is composed of an objective lens, an erecting optical system and an eyepiece, and equipped with a diffractive optical element disposed on an optical path of the erecting optical system (for example, see Japanese Patent Application Laid-Open No. 2001-183590).
However, in the above-described conventional achromatic lens system, since the objective lens is constructed by a glass material, weight-trimming thereof is not sufficient. Moreover, since the objective lens is a single lens, chromatic aberration and spherical aberration cannot be sufficiently corrected.